Lipid suppression in magnetic resonance imaging using multi-coil local B0 field control

Abstract

Lipid suppression in magnetic resonance imaging (“MRI”) is provided on a slice-by-slice basis using tailored local field control that is configured for lipid control for each slice in a planned slice prescription. Only those lipid voxels that fall within the bandwidth of the concurrent RF excitation pulse are targeted. Switched B 0 offset fields are used to improve lipid suppression pulse performance by pushing water and lipids apart in the frequency domain. Multi-coil B 0 shim arrays with rapidly switchable output currents that can be turned on during the lipid suppression pulse may be used. A convex optimization may be used to jointly solve for the shim currents and the lipid suppression pulse center frequency and bandwidth to optimize lipid suppression while minimizing water signal loss.

The invention claimed is: 
     
       1. A method for lipid suppression in magnetic resonance imaging (MRI), the method comprising:
 (a) acquiring k-space data with an MRI system using a pulse sequence comprising a lipid suppression block for suppressing lipid signals and an imaging block for acquiring the k-space data, wherein the lipid suppression block comprises for each slice in a planned slice prescription:
 applying a lipid suppression radio frequency (RF) pulse having a center frequency and a bandwidth tailored for the slice; and 
 generating a B 0  offset field with a shim coil array while applying the lipid suppression RF pulse, the B 0  offset field being tailored for the slice; and 
 
 (b) reconstructing a plurality of images from the k-space data using a computer system, the plurality of images comprising an image for each slice in the planned slice prescription in which lipid signals are substantially suppressed. 
 
     
     
       2. A method for lipid suppression in magnetic resonance imaging (MRI), the method comprising:
 (a) accessing B 0  field map data with a computer system; 
 (b) generating local mask data for each slice in a planned slice prescription using the computer system, wherein the local mask data comprise a lipid mask for each slice in the planned slice prescription; 
 (c) inputting the B 0  field map data and local mask data to a joint optimization algorithm using the computer system, generating shim current settings and lipid suppression pulse parameters as an output; 
 (d) designing a pulse sequence using the computer system, wherein the pulse sequence comprises a lipid suppression block designed based on the shim current settings and lipid suppression pulse parameters; 
 (e) acquiring k-space data from slices in a subject according to the planned slice prescription using an MRI system performing the pulse sequence, wherein the lipid suppression block of the pulse sequence comprises, for each slice in the planned slice prescription:
 applying a spectrally selective radio frequency (RF) pulse using the lipid suppression pulse parameters for the respective slice; 
 generating a B 0  offset field by supplying the shim current settings for the respective slice to a shim coil array; and 
 
 (f) reconstructing a plurality of images from the k-space data, wherein the plurality of images depict the slices in the subject with reduced lipid signals. 
 
     
     
       3. A magnetic resonance imaging system, comprising:
 a magnet system to generating a main magnetic field, B 0 ; 
 a radio frequency (RF) system comprising at least one RF coil; 
 a gradient system comprising at least one gradient coil; 
 a shim system comprising a shim coil array; 
 a processor to:
 receive slice-specific lipid suppression RF pulse parameters; 
 receive slice-specific shim current settings; 
 for a given slice in a planned slice prescription, simultaneously control:
 the at least one RF coil of the RF system to generate a spectrally selective RF pulse according to the slice-specific lipid suppression RF pulse parameters for the given slice; and 
 the shim coil array of the shim system to generate a B 0  offset field according to the slice-specific shim current settings for the given slice. 
 
 
 
     
     
       4. The method of  claim 1 , wherein each B 0  offset field is generated by supplying a shim current to the shim coil array, wherein the shim current is tailored for the respective slice. 
     
     
       5. The method of  claim 1 , wherein the imaging block comprises an echo planar imaging pulse sequence. 
     
     
       6. The method of  claim 1 , wherein the shim coil array comprises a local shim coil array. 
     
     
       7. The method of  claim 1 , wherein the lipid suppression block comprises applying at least one spoiler gradient after the lipid suppression RF pulse to dephase lipid magnetization tipped into a transverse plane by the lipid suppression RF pulse. 
     
     
       8. The method of  claim 2 , wherein the shim current settings and lipid suppression pulse parameters are jointly optimized with the computer system using a joint optimization algorithm. 
     
     
       9. The method of  claim 2 , wherein the lipid suppression pulse parameters comprise a center frequency and a bandwidth. 
     
     
       10. The method of  claim 2 , wherein the pulse sequence comprises an echo planar imaging pulse sequence. 
     
     
       11. The method of  claim 2 , wherein the lipid suppression block comprises applying at least one spoiler gradient after the spectrally selective RF pulse to dephase lipid magnetization tipped into a transverse plane by the spectrally selective RF pulse. 
     
     
       12. The MRI system of  claim 3 , wherein the shim coil array comprises a local shim coil array. 
     
     
       13. The method of  claim 4 , wherein the shim current, center frequency, and bandwidth for each slice are jointly optimized with the computer system using a joint optimization algorithm. 
     
     
       14. The method of  claim 6 , wherein the local shim coil array comprises a plurality of coil that are switchable between RF receive coils and shim coils. 
     
     
       15. The method of  claim 8 , wherein the joint optimization algorithm takes as inputs a B 0  field map and local lipid mask data indicating a lipid mask for each slice in the planned slice prescription. 
     
     
       16. The MRI system of  claim 12 , wherein the at least one RF coil is integrated with the local shim coil array. 
     
     
       17. The method of  claim 13 , wherein the joint optimization algorithm takes as inputs a B 0  field map and local lipid mask data indicating a lipid mask for each slice in the planned slice prescription. 
     
     
       18. The method of  claim 15 , wherein the local mask data are generated with the computer system by simulating a slice-select process for each slice in the planned slice prescription to create the lipid mask for each slice. 
     
     
       19. The MRI system of  claim 16 , wherein the shim coil array comprises coil elements that are switchable between an RF receiver for the at least one RF coil and a shim coil for the local shim coil array. 
     
     
       20. The method of  claim 17 , wherein the local mask data are generated with the computer system by simulating a slice-select process for each slice in the planned slice prescription to create the lipid mask for each slice.